Semiconductor optical amplifiers will play a key role in future wide band optical transmission and optical signal processing systems when employed in an optical preamplification system. In particular, a semiconductor amplifier coupled to an optical detector operates as an optical preamplifier, amplifying the entering optical signal and then converting it to an electrical signal. For very high speed systems, such as those with a data rate greater than a few GBit/sec or a bandwidth greater than a few GHz, optical preamplification offers better performance in terms of signal-to-noise ratio than electrical preamplification, which initially detects the optical signal and then amplifies the detected signal electrically.
Semiconductor optical amplifiers are semiconductor lasers operating below threshold and include low reflectivity facets. Defining P.sub.in as the power (in watts) of the input optical signal and P.sub.out as the power (in watts) of the signal exiting the amplifier, the input and output powers of the amplifier are related by EQU P.sub.out =G P.sub.in ( 1)
The amplifier gain G can be written in terms of the input facet power reflectivity R.sub.1, the output facet power reflectivity R.sub.2, the single pass gain through the amplifier G.sub.s, and the single pass phase shift through the amplifier .beta. as ##EQU1## Clearly, the facet reflectivity can significantly impact the performance of optical amplifiers.
The single pass gain, which can be greater than 30 dB, depends upon the materials, geometry, and doping of the epitaxial layers, the injection current, and the non-radiative losses. However, the actual (or useful) gain from the amplifier is limited by Fabry-Perot resonances, caused by reflections from each facet of the cavity. From a systems perspective, an acceptable amount of ripple in the gain due to these resonances is generally considered to be .ltoreq.3 dB; this allows the amplifier (called a Traveling Wave Amplifier to distinguish it from an amplifier with large resonances, called a Fabry-Perot Amplifier) to be used with conventional semiconductor lasers without preselection for specific wavelengths, and without requiring extreme temperature stabilization of both laser and amplifier. FIG. 1 illustrates the problem of finite reflectivity by showing the gain versus wavelength for two different values of the single pass gain and two different values of the reflectivity. In order to achieve low gain ripple, FIG. 1 shows in curve 11 that the gain for the device with an effective reflectivity R.sub.e =0.5% (where R.sub.e =(R.sub.1 R.sub.2).sup.1/2) is limited to approximately 15 dB. The same device with a lower effective reflectivity of R.sub.e =0.05% can achieve about ten times as much gain, which is approximately 25 dB as shown by curve 13. The other characteristic curves 10 and 12 are for devices with G=15 dB, R.sub.e =0.05% and G=25, R.sub.e =5% respectively.
Conventional approaches to reducing facet reflectivity have focused on the techniques of anti-reflection (AR) coating the facet with a single or multiple layer thin film and tilting the amplifying channel at an angle with respect to the amplifier facet. AR coatings with the required reflectivity (.about.10.sup.-4) on both facets have been fabricated but with great difficulty. A viable way to fabricate conventional semiconductor optical amplifiers has been to combine tilted facets with AR coatings. Another approach presented by Rideout et al. in U.S. Pat. No. 4,872,180 issued Oct. 3, 1989 avoids the use of AR coatings by fabricating an amplifier to have the following features: (1) a bulk regrown end cap region formed at both of the major facet surfaces of the amplifier, (2) an angled waveguide geometry, and (3) index-matching the end cap regions to the waveguide.
A semiconductor amplifier coupled to an optical detector operates as an optical preamplifier by amplifying the entering optical signal and then converting it to an electrical signal. In comparison, electrical preamplification operates by initially detecting the optical signal and then electrically amplifying the detected signal. Operational results of both types of preamplifiers indicate that optical preamplification offers better performance in terms of signal-to-noise ratio for very high speed systems applications such as those with a data rate greater than a few GBit/sec or a bandwidth greater than a few GHz. Consequently, semiconductor optical amplifiers will emerge as integral components in future wide-band optical communication and signal processing systems when optical preamplification is desired.
The copending application entitled "Monolithically Integrated Semiconductor Optical Preamplifier" discloses a monolithically integrated semiconductor optical preamplifier comprising an optical amplifier electrically isolated from an optical detector by a regrown isolation region consisting of an insulating material. The index of refraction of this insulating material is matched to the refractive index of the material constituency of the amplifier region in order to reduce the output facet reflectivity of the amplifier.